SiC material properties give SiC technology advantages over existing Si and III-V technology in the form of very high blocking voltages, high switching frequency, lower on-state and switching losses. SiC has the potential to replace the currently dominating materials in most low- and high-frequency power devices, once the material problems have been mastered. SiC based power electronics will drastically reduce the power losses in most distribution and generation systems for electrical energy as well as in electrical motors.
Semiconductor materials used in almost all electronic semiconductor devices consist of epitaxial layers grown on top of a substrate, sliced from a bulk grown ingot.
SiC cannot be grown from a melt like conventional semiconductors Si and GaAs. Instead vapour phase has to be used. The most common SiC bulk growth technology is based on Physical Vapor Transport (PVT). We have earlier developed an alternative technology based on High Temperature CVD (HTCVD).
Essentially all SiC material sold today is grown by the PVT technique. It is a very simple technique which produces good quality crystals at reasonable yields. However, the technique has also severe limitations and draw backs. It is in essence a container with SiC powder inside it. The container has a lid with a seed attached to it. The container is placed in a furnace such that a thermal gradient is manifest axially with the seed at the coldest spot. At temperatures in excess of 2000° C., silicon carbide starts to sublime appreciably and material transport will occur from the hot source to the colder seed. Growth temperatures are normally around 2100-2400° C. on the seed. Growth pressures are normally around 10 torr which gives an average growth rate around 0.3 mm/h.
Generally, PVT growth starts with a very long heat-up phase followed by a growth phase and a cool down phase that in some cases lasts for a full day. The growth rate during the growth phase also varies as the stoichiometry and axial gradients change. The highest quality material is obtained with very low growth rates (less than 0.3 mm/h) during the growth phase, however if the whole cycle is taken into account, the growth rate will be lower.
The extreme growth temperatures take a high toll on the hot zone consumables and it requires significant amounts of energy to heat and maintain the high temperatures. The sublimation of SiC is not stoichiometric as Si leaves the source first making the growth very Si rich in the beginning, to gradually become more balanced, but end up C rich. This is particularly vexing when high purity semi insulating (SI) crystals are grown as they tend to be n-type at the start and p-type at the end with a very small band of SI material in the middle. The high Si pressures at the start of the growth have severe implications on the way the rest of the SiC crystal grows. Often, polytype flipping caused by the high Si overpressures occur with dislocations and/or micropipes associated with them.
In summary, the PVT technique is a robust technique which gives slow progress in terms of material quality. Higher quality material is very expensive and difficult to produce. The high growth temperature makes the technique expensive in terms of consumables.
HTCVD is a different technique where control is very precise. The HTCVD is also a sublimation technique and hence incorrectly named as a CVD technique at high temperatures. The technique was named before the process was fully understood, hence the unfortunate choice of name. In principle, the technique has replaced the source powder with gases.
Normally, silane and ethylene are used together with helium as a carrier gas. The silane and ethylene decompose and react already in the entrance zone where an injector is placed. The reaction forms micro-crystals of SixCy. These microcrystals are very stable and move into the hot zone where they sublime to form the normal sublimation species Si, Si2C, and SiC2. At the colder seed these species will condense and grow the crystal as in the PVT case. The control is very precise in the HTCVD and the purity is outstanding, however it is hampered by drift issues and poor reproducibility giving rise to poor yields.
In theory, the HTCVD technique should be a cheaper alternative compared to the PVT, however, the current low yields of the process makes it a significantly more expensive technique suitable only for ultra high purity. Growth rates are roughly the same as in the PVT case, roughly 0.3 mm/h. The control makes it also nice in terms of obtaining better structural quality. Indeed, more than 50% of the micropipes originating from the seed are normally closed after a run in a HTCVD-process.
However, both the PVT and HTCVD techniques operate at extreme temperatures where the formation of dislocations and subsequently micropipes is much more probable. Compared to regular CVD where the micropipes more often than not are closed, no new micropipes are formed, and dislocation densities generally decrease with increasing thickness of the layers. More serious is the radial thermal gradients over the growing crystal. If there is no radial gradient over the crystal the risk of obtaining a concave crystal (higher growth rate on the edge than in the center) will be high and is therefore not a desirable situation. In the start up the crystal can also have higher growth rates locally which are caused by variations in the seed attachment. A concave or locally concave crystal will give extremely poor quality material with high stresses and a high density of dislocations. A high radial thermal gradient yielding a convex crystal is not good either as the growth takes place at high temperatures. The structural properties (mainly the lattice spacing) will be different during the growth on material grown on the edge as compared to material grown in the center. When the crystal is cooled down there will be stresses built in which in some cases causes the crystal to crack or it will produce dislocations. The growth of crystals using PVT and to a major extent also when using HTCVD is largely governed by thermal gradients which makes structural imperfections and stresses very hard to avoid. The ideal situation is to grow the crystal without any thermal gradient but to somehow establish a crystal shape that is very weakly convex.
Regarding epigrowth, hydrogen chloride gas (HCl) was first used prior to the growth as an etching process, similarly as in the Si growth. However, already in the year 2000, HCl was used to remove the physicochemical problem, i.e. liquid silicon droplets, which limit the growth rate. The use of chlorinated silicon precursors or simply the addition of HCl or chloride related precursor was suggested in the literature. See, for example, U.S. Pat. No. 7,247,513.
Once chlorinated epitaxy of SiC started to be used, it did not take long before extreme growth rates could be achieved. One of the first studies reported growth rate up to 90 μm/h with methyltrichlorosilane as precursor. HCl was also added to a standard SiC epitaxial growth process using SiH4 and C3H8 to obtain growth rate at about 20 μm/h. Higher growth rates were also obtained, however with a degradation of the morphology. Using SiCl4 and C3H8 as precursors, high growth rates (>100 μm/h) were achieved. With the introduction of HCl in the deposition chamber the growth rate of 4H—SiC epilayer could be increased by a factor of about 20. The introduction of other chlorine containing species has also been tried including chlorosilane gases, such as trichlorosilane. Finally, growth using chloromethane (CH3Cl) as carbon precursor instead of C3H8 has shown an increase of the obtained growth rate with a factor of 2.
There are only two serious attempts of crystal growth using a chlorinated chemistry. Polyakov and coworkers reported the used of chlorinated silicon precursor in conjunction with gaseous hydrocarbon (first with propane: (C3H8), and later with methane (CH4)) for the growth of single crystals (Polyakov et al, Mat. Sc. For. 527, 21 (2006)). They used the advantage of the HTCVD, changing the Si precursor from silane to silicon tetrachloride (SiCl4), split the silicon and carbon flows, and called this technique high temperature Halide Chemical Vapor Deposition (HCVD). Splitting the flow was an elegant trick to retard the decomposition of the SiCl4. The carrier gas for the SiCl4 was Ar and the gas was flowing along the outer conduit of a coaxial injector. In the inner injector the hydrocarbon was passed together with hydrogen. The Ar flowing on the outside would also provide thermal insulation to the hydrocarbon to reduce parasitic deposits of carbon on the walls of the injector. To keep the stability of the 4H polytype the growth temperature was about 2100° C. and the used growth rate 100 μm/h. Growth rate close to 300 μm/h was reported for the 6H polytype. In essence, the technique is elegant but not very practical. None of the issues related to HTCVD have been solved. No thick crystals have been demonstrated and the growth rate of the 4H polytype is very low. Thermal gradients were still manifest and necessary to obtain growth.
Additional documents describing prior art are: U.S.2011285933, U.S. Pat. No. 6,297,522, and WO0043577.
A relevant prior art document is a report of a Research program: E. Janzén, O. Kordina, Chloride-based SiC growth, Linköping University, ref. no. IFM-2010-00154. Page 8, § 4 mentions the outline of a horizontal reactor, but details to make the reactor work practically were not known. A.A further prior art document is: S Leone, F C Beyer, A Henry, C Hemmingsson, O Kordina, and E Janzén. Chloride-based SiC epitaxial growth toward low temperature bulk growth. Crystal Growth and Design, 10(8):3743-3751, 2010.